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Category Time & Date

Start Date

Select the beginning of your date range

End Date

Select the end of your date range

Include end day in count

Select two dates and click Calculate

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About

Calculating the interval between two dates manually invites errors that compound in contracts, project timelines, and financial instruments. A miscounted month can shift a payment deadline; a forgotten leap day invalidates an entire schedule. This calculator computes the exact duration using iterative month-stepping rather than the flawed 30.44-day average, correctly handling February's variable length and year boundaries. It returns Y years, M months, D days alongside total days, weeks, business days (excluding weekends), and granular breakdowns to hours and minutes. ISO 8601 week numbers for both dates are included for sprint planning and fiscal reporting.

The business day calculation excludes Saturdays and Sundays but not public holidays, which vary by jurisdiction. For legal documents or SLA calculations, verify against your local holiday calendar. The algorithm assumes the Gregorian calendar; dates before October 15, 1582 will produce mathematically correct but historically anachronistic results.

Formulas

The primary calculation uses iterative month-stepping to accurately handle variable month lengths:

D_total = t_end − t_start86400000

where t represents the Unix timestamp in milliseconds. For the calendar breakdown:

Y = floor(months_diff ÷ 12)

M = months_diff mod 12

Leap year detection follows the Gregorian rule:

isLeap = (y mod 4 = 0 ∧ y mod 100 ≠ 0) ∨ (y mod 400 = 0)

ISO 8601 week number calculation:

W = floor(ordinalDay − weekday + 107)

where ordinalDay is the day of the year (1 - 366) and weekday uses Monday = 1 through Sunday = 7.

Reference Data

Time Unit	Equivalent	Notes
1 minute	60 seconds	SI base conversion
1 hour	60 minutes	3,600 seconds
1 day	24 hours	86,400 seconds
1 week	7 days	168 hours
1 fortnight	14 days	Common in UK payroll
1 month (avg)	30.437 days	Gregorian average; actual 28 - 31
1 quarter	3 months	~91 days; fiscal reporting
1 year (common)	365 days	8,760 hours
1 year (leap)	366 days	Divisible by 4, exceptions apply
1 decade	10 years	~3,652 days
1 century	100 years	24 leap years typically
ISO Week	1 - 53	Starts Monday; Week 1 contains Jan 4
Julian Day Number	Continuous count	Day 0 = Jan 1, 4713 BC
Unix Epoch	Jan 1, 1970 00:00 UTC	Seconds since epoch
Business Days/Year	~252	US average; excludes weekends + ~10 holidays
Work Hours/Year	~2,080	52 weeks × 40 hrs
Days in February	28 or 29	Leap year rule determines
Leap Year Rule	Divisible by 4	Except centuries not divisible by 400
Gregorian Adoption	Oct 15, 1582	10 days skipped from Julian

Frequently Asked Questions

The calculator uses iterative month-stepping rather than a fixed 30.44-day average. It advances month-by-month from the start date, correctly accounting for February's 28 or 29 days and months with 30 or 31 days. For example, January 31 to February 28 is exactly 1 month (not 28 days), while January 31 to March 1 is 1 month and 1 day in a non-leap year.

No. The business day count excludes only Saturdays and Sundays. Public holidays vary by country, region, and even industry. For precise legal or contractual calculations, subtract your applicable holidays manually. The US averages approximately 10 federal holidays per year, reducing annual business days from ~261 to ~251.

Date range calculations can be inclusive or exclusive of the end date. This calculator offers an "Include end day" toggle. When enabled, both the start and end dates count (e.g., January 1 to January 3 = 3 days). When disabled, the end date is excluded (= 2 days). Contracts typically specify "through" (inclusive) or "until" (exclusive).

The calculator uses the proleptic Gregorian calendar, extending Gregorian rules backward. Historically, most regions used the Julian calendar before October 15, 1582, which drifted ~10 days. Results for pre-1582 dates are mathematically consistent but do not reflect historical reality. For historical research, adjust for the Julian-Gregorian transition.

ISO 8601 defines week 1 as the week containing January 4th (or equivalently, the first week with a Thursday). Weeks start on Monday. This standard is used in European business contexts, software development sprints, and fiscal reporting. A year can have 52 or 53 ISO weeks. December 31 may fall in week 1 of the following year.

Yes. The JavaScript Date object reliably handles dates from approximately 271,821 BC to 275,760 AD. Practical limits arise from leap year accumulation (~24 leap years per century) and the iterative month calculation, but spans of several centuries compute correctly in under 50 milliseconds.